It has heretofore been proposed, for example, as shown in U.S. Pat. Nos. 2,302,449; 2,302,450; 3,239,061 and 4,152,265 to make a filter apparatus in which the filter bed is supported on a generally horizontal porous support in a tank and partitions provided at spaced locations along the tank to separate the bed into a plurality of filter cells above the porous support and to divide the space below the porous support into a plurality of filtrate compartments that communicate at one end through a filtrate port with a filtrate launder that extends along the length of the tank. In such filter apparatus, the fluid to be filtered is supplied to the tank above the bed of filter material and passes downwardly through the bed of filter material into the filtrate compartments and out of the filtrate compartments into the filtrate launder to a discharge outlet. The bed of filter material is progressively cleaned by a carriage mounted for movement along the bed and having a backwash head movable through the filtrate launder sequentially into communication with the filtrate ports to supply backwash fluid to the filtrate compartments at the underside of the porous support and a collector hood movable along a path over the top of the beds to collect the backwash effluent. Backwash water is supplied to the backwash head by a backwash pump and a waste water pump is connected to the collector hood for pumping waste water from the hood into a waste water trough.
In order to obtain efficient cleaning of the bed it is desirable to adjust the backwash to a flow rate which will loosen the slit and solids in the bed without causing removal of filter media. It is also desirable to adjust the effluent flow rate from the hood in relation to the backwash flow rate. If the effluent flow rate is too low, the backwash water tends to blow the silt and solids out of the hood and into the tank. On the other hand, if the effluent flow rate is too high, excessive water is withdrawn from the tank above the bed and passed to drain with the waste water. This adjustment of the backwash and effluent flow rates is desirable not only at the time of installation of the filter, but also periodically after installation to compensate for variation in the delivery of one or the other of the pumps as may be caused, for example by wear in the pump or by debris accumulation in the pumps and conduits. Measurement and proper adjustment of the backwash and effluent flow rates has presented a continuing problem in the field.